The present invention relates to a method of operating etching baths whereby a metal workpiece to be etched is dipped in an etching bath containing a medium, the metal is oxidized by a chemical reaction between metal and etching medium and converted into a soluble complex, a resulting mixture obtained is separated by removing the excess etching medium from the complex, the recovered etching medium is reused to operate etching baths, and the complex is subjected to a recovery process.
European Patent Application A 0 465 822.
The etching bath described therein is an alkaline etching bath in which workpiece made of aluminum are dissolved using a sodium hydroxide solution according to the following equation:
2Al+2NaOH+6 H2O 2NaAl(OH)4+3H2
Thus aluminum is oxidized and converted into a water-soluble aluminate complex Al(OH)4.
In the method described in European Patent Application A 0 465 822 for recovering sodium hydroxide and obtaining aluminum hydroxide, the etching bath mixture having a high concentration of metal ion complex is treated so that a dialysis step is initially performed. In this dialysis step, most of the sodium hydroxide is separated from the aluminate complex. The sodium hydroxide solution obtained in the dialysis step is still sufficiently concentrated so it can be recycled to an etching bath. The aluminate solution is, however, considerably diluted by the dialysis step. The concentration is approximately one-half of the original concentration.
The process of recovery of the complex includes hydrolyzing the aluminate complex into insoluble aluminum hydroxide and a sodium hydroxide solution by adding water, which occurs according to the following equation:
H2O
NaAl(OH)4←xe2x86x92NaOH+Al(OH)3
The aluminum hydroxide that precipitates represents a valuable material, which is supplied to the aluminum industry, for example; it approximately corresponds to the material obtained in the known Bayer process.
The disadvantage of the above-mentioned method is that large amounts of water are needed for the dialysis step, i.e., for the diffusion dialytic removal of sodium hydroxide, in order to maintain the concentration gradient needed for the operation of the dialysis. Dialysis rates are extremely low, and a very high membrane surface is needed to convert the required amounts.
In order to recover the complex in an industrially feasible manner, very high aluminate concentrations must be initially achieved in the etching bath, since otherwise the solution becomes diluted during the dialysis step to the point that proper recovery is no longer possible.
It has, however, been found that it is unfavorable for the etching results if very high concentrations of metal complex build up in the etching bath, since this considerably reduces the etching speed. If the concentration in the-etching bath comes close to the saturation limit of the metal complex during operation, there is the risk that improper control may cause precipitation in the bath, which is by no means desirable, since the workpieces to be machined are then contaminated and expensive cleaning steps must be subsequently performed.
U.S. patent application Ser. No. 5,141,610 describes a method of performing the separation of the etching medium by electrodialysis in the recovery of acidic or alkaline etching baths. In electrodialysis, an electric field is applied across the membrane, in which the dialysis membrane absorbs a sodium hydroxide solution, for example, on one side and releases it on the other side. Electrolytic processes take place on the electrodes.
This method requires less membrane surface; however, it requires a high amount of power, and the electrodes required for the electrolysis are attacked by the highly corrosive bath medium. This method is not suitable for economically processing large amounts of etching bath liquid in the order of hundreds of cubic meters.
Processing economically on an industrial scale is an aspect of the present invention.
World Patent 01 670 Al describes a method for processing aqueous, metal ion-containing waste solutions. The separation of process products and their removal from the solvent (water) is performed by combining reverse osmosis and a xe2x80x9cwater splittingxe2x80x9d process, such as electrodialysis. The process products are separated from one another by ion-selective electrodialysis and water is separated from the process products using reverse osmosis; the sequence of operations is unimportant. The different process streams are at least partially recycled.
However, reverse osmosis is not ion-selective and therefore can only be used for separating process products in combination with another process such as electrodialysis. Combining two processes involves high process costs. The problems of electrodialysis have been mentioned previously.
Etching is understood in the context of the present invention not as light superficial etching or corroding of a piece of metal, but well-defined and substantial removal of material, such as performed in machining.
In the aerospace industry, large components made of aluminum are used; for example, aircraft fuselages are largely made of aluminum. Due to the manufacturing tolerances in the molding processes, for example, for forming aircraft fuselage linings, the premolded parts must be subjected to chemical etching in order to remove large amounts of material, i.e., aluminum, often only from certain areas. In the aerospace industry, component weight is very important, so that it is customary to chemically etch molded components, which are too heavy due to manufacturing tolerances, to a certain weight.
Accordingly, etching baths are very large. Volumes of 60 m3 are possible for handling such large components. Consequently large amounts of etching baths must be recovered.
Therefore, an object of the present invention is to operate an etching bath so that favorable and easy-to-control bath conditions are always ensured, yet etching bath recovery requires simple equipment and has a low power consumption.
The present invention provides a method of operating etching baths and a device for carrying out the method in which a mixture of dissolved complexed metal ion and etching medium is removed from the etching bath at a concentration of the dissolved complexed metal ion that is far below its saturation to limit.
A nanofiltration separation is performed in which the etching medium is separated from the mixture using the principle of reverse osmosis, with the concentration of the complexed metal ion in the residue being increased simultaneously.
Thus, according to a the present invention, the mixture of dissolved complexed metal ion and etching medium is removed when the complexed metal ion concentration is still relatively low. As mentioned before, it has been determined that well-defined etching conditions that are easy to control can be achieved at relatively low dissolved metal complex concentrations, which become increasingly unfavorable and difficult to control as the concentration increases. The present invention provides the possibility of an economical subsequent recovery because the concentration of complexed metal ion in the residue increases simultaneously due to the separation by nanofiltration.
In nanofiltration, the medium to be filtered is subjected to pressure and ion-selective membrane separation occurs. The synonyms hyperfiltration and low-pressure reverse osmosis are also used for the term nanofiltration. Small molecules, such as sodium hydroxide or water, for example, can easily pass through the membrane, while larger molecules, such as those of the metal complex, only pass through the membrane with difficulty. Therefore, both water and etching medium, for example, a sodium hydroxide solution, can be separated from the etching bath mixture, whereby the metal complex concentration in the remaining solution increases.
When the etching bath is operated according to the present invention, the etching bath is not left to attain a level of high metal complex concentration that allows economical subsequent recovery, but it is removed from the etching bath at very low concentrations. In the subsequent nanofiltration, the etching medium and water are separated and thus the metal complex is concentrated at the same time so that the complex can be economically recovered.
The object is achieved completely in this way.
In another embodiment of the present invention, in which etching medium is released again as the complex is recovered, the resulting solution is also sent to nanofiltration.
Returning to the example of recovery of a sodium aluminate complex, a sodium hydroxide solution is obtained again. This means that the aqueous filtrate contains sodium hydroxide solution and residual amounts of sodium aluminate complex. By supplying this solution to the solution to be nanofiltered, the sodium hydroxide that was bound in the aluminate complex in the previous nanofiltration can also be separated and recycled to the etching bath. This makes it possible to recycle almost all the necessary sodium hydroxide solution, which results in considerable economical and environmental advantages in industrial systems.
In another embodiment of the present invention, in which the complex is recovered by diluting with water and precipitating and separating the metal as a hydroxide, water addition is controlled so that the filtrate resulting from the separation compensates for the evaporation losses of the etching bath.
Etching baths are often operated at high temperatures, for example in the range of 75-80xc2x0 C., so that a considerable amount of water gradually evaporates from the etching bath. By adding an appropriately diluted filtrate from the recovery process of the complex, appropriate amounts of water can be supplied to the nanofiltration, which pass through the nanofiltration membrane together with the sodium hydroxide solution and thus ensure sufficient water is refilled into the etching bath.
In another embodiment of the present invention, the mixture of dissolved complexed metal ion and etching medium is removed from the etching bath continuously or batchwise, depending on the size of the system and the control method used.
In another embodiment of the present invention, namely when operating alkaline etching baths for dissolving aluminum, the mixture of hydroxide solution and aluminate complex is removed at aluminum bath concentrations in the range of 5-20 g/l aluminum. This measure offers the advantage that the bath is operated at very low aluminum concentrations. At very low aluminum concentrations in the bath almost pure sodium hydroxide solution is present, which is so concentrated and aggressive that, for example, when very large parts are dipped into it, no uniform etching effect can be achieved, since the areas dipped first are etched for a longer time and therefore more strongly than those dipped later. At very high aluminum concentrations in the etching bath, the etching rate strongly decreases, creating the risk of overconcentration points, so that precipitation may occur.
In another embodiment of the present invention, the hydroxide solution concentration in the etching bath is in the range of 110-150 g/l sodium hydroxide. The sodium hydroxide obtained in nanofiltration is continuously recycled so that the concentration is maintained in this advantageous range, which allows an optimum and well-defined, and thus easily controlled etching result to be achieved.
In another embodiment of the present invention, nanofiltration is performed so that aluminum is concentrated to 100 g/l.
By increasing concentration to this high range, a supersaturated metal complex solution is obtained, which can be economically recovered. Spontaneous crystallization takes place in this solution.
While basically the dissolution of aluminum was described above in order to explain alkaline etching baths, of course obviously other metals that dissolve and form corresponding complexes in alkaline media, such as zinc, can also be etched.
A similar method can also be used in operating acidic etching baths, where the anion is converted to a complex in the acid, for example, chloro-complexes are formed during the dissolution of hydrochloric acid. Such complexes can also be recovered by diluting with water or by making alkali, and the corresponding metal hydroxides can be precipitated.